In recent years, development of a blue laser which can realize huge recording density has progressed rapidly. Moreover, development of a write-once-read-many optical recording medium for the blue laser has been underway. Above all, there has been a strong demand on development of a write-once-read-many medium of a dye coating type which can be efficiently produced at relatively low cost. In a conventional dye coating type write-once-read-many optical recording medium, a recording pit is formed by applying a laser beam to a recording layer of an organic compound mainly made of dye, generally resulting in optical changes (in a refractive index or an absorption rate) attributable to decomposition or alteration of the organic compound. A recording pit portion is normally associated with not only the optical changes but also deformation due to a change in the volume of the recording layer, formation of a mixture portion of a substrate and a dye due to heat generation, substrate deformation (mainly bulging caused by substrate expansion), and the like (see patent documents 1, 2, 3, and 4).
Optical behavior of the organic compound used in the recording layer with respect to a laser wavelength used for recording and reading as well as thermal behavior such as decomposition, sublimation and accompanying heat generation are important factors in the formation of a good recording pit. Therefore, as to the organic compound used in the recording layer, it is required to select a material having appropriate optical properties and decomposition behavior.
The conventional write-once-read-many medium, especially a CD-R and a DVD-R are originally intended to maintain read-compatibility with a read-only recording medium (a ROM medium) which has concave pits formed in advance on a substrate covered with a reflective film such as Al, Ag and Au. Moreover, those media are intended to realize reflectivity of approximately 60% or more as well as high modulation over approximately 60%. First, in order to obtain high reflectivity in an unrecorded state, optical properties of the recording layer are specified. Normally, in the unrecorded state, a refractive index n is required to be about 2 or more and an extinction coefficient is required to be about 0.01 to 0.3 (see patent documents 5 and 6).
With the recording layer mainly made of dye, it is difficult to achieve high modulation of 60% or more only by use of changes in the optical properties caused by recording. Specifically, changes in the refractive index n and an extinction coefficient k are limited in the dye that is an organic substance. Thus, changes in the reflectivity in a planar state are limited.
Accordingly, there has been utilized a method of apparently increasing the change in reflectivity (reduction in reflectivity) in the recorded pit portion by use of an interference effect of reflected light due to a phase difference in the reflected light between the recorded pit portion and an unrecorded part. Specifically, there has been used the same principle as a phase difference pit such as the ROM medium. In the case of an organic recording layer having a smaller change in refractive index than an inorganic one, it has been reported that it is rather advantageous to mainly use the change in reflectivity due to the phase difference (see patent document 7). Moreover, there have been conducted studies comprehensively considering the recording principles described above (see nonpatent document 1).
The portion recorded as described above (sometimes called a recording mark part) will be hereinafter called a recording pit, a recorded pit portion or a recording pit portion regardless of its physical shape.
FIG. 1 is a view showing a write-once-read-many medium (optical recording medium 10) with a conventional configuration, which has a recording layer mainly made of dye. As shown in FIG. 1, the optical recording medium 10 is formed by sequentially forming at least a recording layer 12, a reflective layer 13 and a protective coating layer 14 in this order on a substrate 11 having grooves formed thereon. By use of an objective lens 18, a recording/reading light beam 17 is made incident through the substrate 11 on the recording layer 12. As a thickness of the substrate 11, 1.2 mm (CD) or 0.6 mm (DVD) is usually adopted. Moreover, a recording pit is formed in a portion of a substrate groove part 16, which is normally called a groove, on a near side when viewed from a plane 19 on which the recording/reading light beam 17 is made incident. The recording pit is not formed in a substrate inter-groove (land) part 15 on a far side.
In the publicly known documents described above, a change in phase difference is increased by making a change in refractive index as much as possible between before and after recording of the recording layer 12 containing dye. Meanwhile, it has been also reported that a change in a shape of a recorded pit portion, in other words, effects such as a local change in groove geometry in the recorded pit portion formed in the groove (a groove depth is equivalently changed by bulging or collapse of the substrate 11) and a change in a film thickness (an equivalent change in the film thickness by expansion or contraction of the recording layer 12) contribute to the change in phase difference.
In the recording principles as described above, reflectivity at the time of unrecording is enhanced, and the organic compound is decomposed by laser irradiation to cause a significant change in refractive index (thereby obtaining high modulation). Thus, normally, a recording/reading light wavelength is selected to be positioned at an edge on a long wavelength side of a large absorption band. This is because, at the edge on the long wavelength side of the large absorption band, there is obtained a wavelength region which has an appropriate extinction coefficient and a large refractive index.
However, there has not been found a material having optical properties equivalent to conventional ones with respect to a blue laser wavelength. Particularly, in the vicinity of 405 nm that is the center of an oscillation wavelength of a blue semiconductor laser, which has been currently put to practical use, any organic compound having the same optical constant as that required for the recording layer of the conventional write-once-read-many optical recording medium is hardly obtained. Such an organic compound is still under investigation. Furthermore, in the conventional write-once-read-many optical recording medium having the dye recording layer, there exists a main absorption band of the dye in the vicinity of the recording/reading light wavelength. Thus, dependence of its optical constant on wavelength is significant (the optical constant significantly varies depending on the wavelength). Consequently, there is a problem that, in response to individual differences of lasers or fluctuations of the recording/reading light wavelength due to a change in an environmental temperature and the like, recording sensitivity, modulation, recording characteristics such as jitter and an error rate, reflectivity, and the like are significantly changed.
For example, there has been reported an idea of recording using a dye recording layer having absorption in the vicinity of 405 nm. However, the dye used therein is required to have the same optical characteristics and functions as those of the conventional one. Therefore, such an idea depends absolutely on search for and discovery of a high-performance dye (see patent documents 8 and 9). Next, it has been reported that, in the write-once-read-many optical recording medium 10 using the conventional recording layer 12 mainly made of dye, as shown in FIG. 1, it is required to appropriately control the groove geometry and a distribution of thicknesses of the substrate groove part 16 and the substrate inter-groove (land) part 15 in the recording layer 12 (see patent documents 10, 11, and 12).
Specifically, from the viewpoint of securing high reflectivity as described above, only a dye having a relatively small extinction coefficient (about from 0.01 to 0.3) with respect to the recording/reading light wavelength can be used. Thus, it is impossible to reduce the thickness of the recording layer 12 in order to obtain light absorption required for recording in the recording layer 12 and to increase a change in phase difference between before and after recording. As a result, as the thickness of the recording layer 12, a thickness of about λ/(2ns) (ns is a refractive index of the substrate 11) is usually used. Moreover, it is required to use the substrate 11 having deep grooves in order to reduce crosstalk by burying the dye used for the recording layer 12 in the grooves. The recording layer 12 containing the dye is usually formed by use of a spin-coating method (a coating method). Thus, it is rather advantageous to use the thicker the recording layer 12 in the groove part by burying the dye in the deep grooves. Meanwhile, by using the coating method, there occurs a difference in the thickness of the recording layer between the substrate groove part 16 and the substrate inter-groove (land) part 15. However, occurrence of the difference in the thickness of the recording layer is effective in stably obtaining a tracking servo signal even by use of the deep grooves.
Specifically; concerning the groove geometry defined on the surface of the substrate 11 shown in FIG. 1 and groove geometry defined on an interface between the recording layer 12 and the reflective layer 13, both of signal characteristics and tracking signal characteristics in the recorded pit portion cannot be maintained well unless both of the groove geometries described above are maintained to have appropriate values. Usually, it is required to set a depth of the groove around λ/(2ns) (λ is a wavelength of the recording/reading light beam 17 and ns is a refractive index of the substrate 11). The depth is set about 200 nm for a CD-R and about 150 nm for a DVD-R. It has become very difficult to form the substrate 11 having such deep grooves, which is a factor in lowering quality of the optical recording medium 10.
Particularly, in an optical recording medium using a blue laser beam, when λ=405 nm, deep grooves of about 100 nm are required. Meanwhile, a track pitch is often set to from 0.2 μm to 0.4 μm in order to realize high density. However, it is even more difficult to form such deep grooves at the narrow track pitch. In reality, mass production is almost impossible by use of conventional polycarbonate resin. Specifically, as to the medium using the blue laser beam, mass production thereof is likely to become difficult with a conventional configuration.
Furthermore, many examples in the patent documents described above are the example of FIG. 1 showing the conventional disk configuration. Meanwhile, in order to realize high-density recording using a blue laser, a configuration, so-called surface incidence, has attracted attention. There has been reported a configuration using an inorganic material recording layer such as a phase change recording layer (see nonpatent document 3). In the configuration called the surface incidence, contrary to the conventional case, at least a reflective film, a recording layer and a cover layer are sequentially formed in this order on a substrate having grooves formed therein. A focused laser beam for recording/reading is made incident through the cover layer to be applied on the recording layer. Usually, for a so-called Blu-Ray disk, a thickness of the cover layer is set to about 100 μm (nonpatent document 9). The reason why recording/reading light is made incident from such a thin cover layer side is because, as an objective lens for focusing the light, one having a numerical aperture (NA) higher than that of the conventional case (normally from 0.7 to 0.9, and 0.85 for the Blu-Ray disk) is used. In the case where an objective lens having a high NA (numerical aperture) is used, it is required to set the cover layer as thin as about 100 μm in order to reduce an influence of aberration due to the thickness of the cover layer. There have been reported a number of examples related to such blue wavelength recording and surface incidence layer configuration (see nonpatent document 4 and patent documents 13 to 24). Moreover, there have also been many reports concerning the related technologies (see nonpatent documents 5 to 8 and patent documents 25 to 36).
Nonpatent document 1: “Proceedings of International Symposium on Optical Memory”, (U.S.A.), Vol. 4, 1991, p. 99-108
Nonpatent document 2: “Japanese Journal of Applied Physics”, (Japan), Vol. 42, 2003, p. 834-840
Nonpatent document 3: “Proceedings of SPIE”, (U.S.A.), Vol. 4342, 2002, p. 168-177
Nonpatent document 4: “Japanese Journal of Applied Physics”, (Japan), Vol. 42, 2003, p. 1056-1058
Nonpatent document 5: “Compact disc dokuhon”, written by Heitaro Nakajima and Hiroshi Ogawa, 3rd revised version, Ohmsha, 1996, p. 168
Nonpatent document 6: “Japanese Journal of Applied Physics”, (Japan), Vol. 42, 2003, p. 914-918
Nonpatent document 7: “Japanese Journal of Applied Physics”, (Japan), Vol. 39, 2000, p. 775-778
Nonpatent document 8: “Japanese Journal of Applied Physics”, (Japan), Vol. 42, 2003, p. 912-914
Nonpatent document 9: “Optical disc kaitaishinsho”, Edited by Nikkei Electronics, Nikkei BP Inc., 2003, Chapter 3
Nonpatent document 10: “Spectroellipsometry”, Maruzen Publishing Company, 2003, Chapter 5
Nonpatent document 11: Secchakuzai to secchaku gijutsu nyumon”, written by Alphonsus V. Pocius, translated by Hiroshi Mizumachi and Hirokuni Ono, Nikkan Kogyo Shimbunsha, 1999
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